New tires affecting gas mileage on WRX

here in NJ. I think it was later, like the end of October though. It drove me nuts, I thought my brakes were dragging etc. Just a thought...

I have a MY02 TS, not a WRX, but I've got the 16" WRX wheels and tires from new, and last year changed over to 950's, as you have. No change to the gas mileage I've been able to detect, and I also keep pretty close track of usage.

Larry Van Wormer

must have gotten a load of California gas

if the diameter is the same and otehr things are the same, gas is to be suspected.

I noticed a mpg drop when I switched out my stock 16s for STi take offs (17"BBS w/RE070s) with basically 0 miles on them. After a couple thousand the tires seemed to get "broken-in" and my mileage went back to where it was. This was also very noticable in rolling resistance in my driveway. I normally push my car out of the garage to wash it and save starting it up, but found I could barely push it after the wheel/tire upgrade. Now it is back to the normal rolling resistance as before.

Go figure,

Steve.

I made the exact same tire change you did. I've had my RE950's on for

My mileage did not decrease when I switched tires. I've averaged 19.14 mpg since I started keeping track - but that includes 3000 miles of pulling a trailer. My average tank is ~21mpg. It's actually about 1 mpg better then it was with the old tires. I also run ~2psi higher than the doorjam label.

I'm guessing that somehow your alignment got messed up - either hitting a bump or even at the tire shop. If there's too much toe, handling can still be great, but there's a lot more resistance. Just something to check maybe.

- Byron '02 WRX wagon

Not with my Subie, but an almost exactly 10% loss with a change of tires on another vehicle (based on very good records.) Same as your experience: going to "stickier" tires.

Rick

I'm thinking the alignment thing too. If you have just a tad too much toe-in, & you put on sticker tires, then the rolling resistance will increase & cause the drop in mileage. ~ Paul aka "Tha Driver"

Giggle Cream - it makes dessert *funny*!

Many people, myself included, run tire pressures 6-10 pounds above the Subaru recommended pressures. As long as you stay below the maximum molded into the side of the tire, checked when cold, there won't be a safety issue. I run my 205/55-16's at 38-40 psi front, 36-38 psi rear. Ride is not as soft, but fuel efficiency and steering feel are improved.

That said, the previous comments about an alignment check are good advice.

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I don't know about that. What about premature wear down the center and a reduced contact patch?

Big time. After wearing out the RE-92's on my '02 OBS and swapping on some RE-950's, my city mileage immediately dropped from 24mpg to 22mpg. Interestingly, as Wormer mentioned, now that I've got over 30K miles on the RE-950's the mileage has come back up to 24.

The RE-92 is a LOW ROLLING RESISTANCE TIRE. It has flexible cord structure in the sidewalls, whereas the RE-950 is a very staunch-feeling tire for an H speed rating.

My main beef is the the negative camber built into the rear suspension geometry, which causes the inside tread-blocks to develop an extremely noisy feathered edge.

BTW, you might want to try a little higher pressure (38-40 psi) with this tire. That way you'll use less gasoline to heat buildup in the sidewalls, and the steering feel will open up too.

At the recommended pressures, my tires roll over onto their shoulders easily during hard cornering, and this wears the ouside edges of the tires at an alarming rate. I've put chalk on the sidewalls, then increased pressure little by little until the chalk doesn't get rubbed off while driving on twisty roads. That's how I found the pressures I prefer.

In spite of that, on my WRX, while running -1 degrees camber in the front, and 8 psi more pressure than recommended, I still get faster wear on the outside edges than inside or center. This has been true for 4 different sets of tires I've had on this car.

If I drove more conservatively, this would probably be less of an issue. If I were to always drive as if there was (for instance) an infant in the car, I would slow down more for curves and corners, and my tires might well wear more in the center.

As far as contact patch size, it changes dynamically when you drive, and I use the criteria above and don't worry about it. After all, the biggest contact patch is on the flattest tire.

I think I'd better put on my asbestos underwear now.

Hi,

Have you tried playing with the toe to offset some of that camber wear? I'm thinking back to mid-70s when I got my first front wheel drive car, and out of habit, even though it was brand new, took it right in to my then favorite alignment shop just to be checked.

The fellow reset it with a tiny bit of toe out, despite the book spec for some toe in. His reasoning had to do with whatever oddities go on with front wheel drive, particularly in cornering, though I'm not sure I understood exactly what he was getting at. I traded that car about 32k miles later, with the tires still in great shape. I got another FWD, repeated the drill, and that car didn't see the inside of a tire shop until it had 107k miles on the clock (five tire X rotation pattern, so each tire had ~85k miles. Not bad for 155-13s!) I wore out two sets (four tires only) of the same tires in the next ~80k, with the alignment set back to the book cuz I'd moved and didn't know exactly what to tell the new alignment guy. So my original guy musta known something...

I don't know if the AWD shares any of the "characteristics" of FWD, but it might be worth asking around.

Rick

Rick, similar to deviating from the manufacturer-recommended tire pressures, the alignment specs can be deviated from for the purpose of trading one characteristic for another.

The slight toe-in setting is usually desireable for reducing "lane wander", and to provide a on more on-center feel.

Also, as the bushings and tie-rod ends begin to develop wear on a FWD car, the actual toe and camber settings will move around with varying throttle input.

Outside of the impressive tire mileage you got, what's the advantage to toe-out, if not recommended by the mfr?

Yes, I have played with the toe. At 1/8" toe-out, I was getting some slight feathering across the whole tire, more pronounced at the outside edges. Right now the toe is set to zero. It's been there for about 6 months, and tire wear looks good, but the steering was more lively and responsive when I had the toe-out. Oh well.

Part of the reason for the negative camber is that I always seem to wear the outside edges of the tires out first. I figure that the higher tire pressures tend to wear out the center and preserve the edges, and the negative camber tends to wear out the inside, so that compensates for me scrubbing all that rubber off the outside edges in hard corners.

Many of the more hard-core WRX guys run camber at -2 or -3 degrees. I chose my alignment settings primarily for tire wear, and I think they are as good as can be for that. It's my driving that's wearing the outside edges. Something to reduce body roll might help, but I've not decided how far I want go with that.

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I can't ~really~ say there was any advantage, other than tire mileage. The points Danny made about lane "wander" and on-center feel are well taken, but I didn't really notice any negative change in either. In fact, while the car was aligned with the "toe out" situation, it ~seemed~ to be more stable than later when it was returned to the book specs. My unofficial "stability" test at the time was driving with one knee while pouring coffee from a Thermos (don't try this at home, kids!) so don't take what I'm saying as anything but anecdotal. You'd want to talk with a knowledgeable alignment guy for better input than mine.

Rick